The invention relates to thermoplastic products having slip resistant surface properties. The invention particularly applies to thermoformed pickup truck bed liners having an increased coefficient of friction on load bearing and cargo carrying surfaces thereof. The invention further relates to a variety of other practical and industrial applications such as pallets, totes and trays for packaging and shipping, sport and recreational products, and walking surfaces such as marine docks and decks, swimming pool surfaces, showers, bathtubs, walks, stairs, and floor mats.
Thermoplastic materials are being used to manufacture a growing variety of products. Thermoplastics can be conveniently and economically formed by various methods including thermoforming, injection molding, blow molding, compression molding, rotational molding, and extruding, for example. Products made of thermoplastics are durable, economical, and adaptable to many different applications. One disadvantage that has plagued many thermoplastic products is a relatively low surface coefficient of friction. In other words, the thermoplastic surfaces are too slippery to be acceptable in some applications. The thermoplastic surfaces become especially slippery when wet, and thus may present dangerous conditions when thermoplastic products are used in an outside environment, e.g. pickup truck bed liners, or used in typically wet conditions, e.g. swimming pool or spa areas and marine docks and the like, and especially when the thermoplastic materials form walking surfaces, e.g. steps, floor mats, runners, and the like.
A particular thermoplastic material that has been proved economical and suitable for making many different products is high density polyethylene (HDPE). However, high density polyethylene exhibits a rather low coefficient of friction, especially when wet, so it particularly suffers the disadvantages of slipperiness mentioned above. This problem is well recognized in the field, and various attempts have been made to correct it. One approach for increasing the surface coefficient of friction involves mechanically forming a rough surface texture on the thermoplastic material, for example by embossing the surface of the product as it is being molded. Disadvantages of this approach are that the embossing increases the cost and complexity of the molding or other forming operation, the embossed surface textures have been found to perform inadequately when wet, and the embossed surface textures can wear down with use, whereby the coefficient of friction is again reduced to that of the smooth ordinary HDPE.
Another approach for trying to improve the surface coefficient of friction is to apply a surface coating onto the polyethylene material. The difficulty with such an approach is that the HDPE is rather inert and molecularly non-active, so that conventional antislip coating materials do not adhere sufficiently to the HDPE surface. Thus, the durability and wearability of the surface is inadequate, because the coating materials peel or flake off of the HDPE. In order to provide sufficient adhesion, prior efforts have involved forming a continuous film of an antislip material onto the HDPE surface, for example by coextruding or coforming the antislip film with the HDPE material. By providing such a continuous film of antislip material, the surface area of contact adhesion is maximized, and the locations at which peeling, separation, or abrasion of the antislip material from the underlying HDPE material are minimized. Unfortunately, the use of a continuous film of an antislip material increases the cost of the product, limits the types of forming or molding processes that can be used to manufacture the product, and also increases the cost and complexity of carrying out the forming method.
An example of the above discussed attempts to use a continuous film of nonslip material on an HDPE base material is represented by conventional bedliners for pickup truck cargo carrying areas. Essentially all truck bed liners are thermoformed from high density polyethylene (HDPE) sheets. Other materials such as ABS, synthetic rubber plastic, and other thermoplastic polymers are sometimes used. The thermoplastic sheets are heated to a thermoforming temperature in a thermoforming oven and are deformed by ambient air pressure after application of vacuum on one side in a thermoforming mold to produce the desired product. Many products other than truck bed liners are also manufactured by this process.
U.S. Pat. No. 4,693,507 (Dresen et al.) describes a truck bed liner with antislip surface properties. The disclosure of U.S. Pat. No. 4,693,507 is incorporated herein by reference. An increased coefficient of friction on the liner surface is achieved by applying a continuous film or layer of elastomeric material over the HDPE sheet. The thickness of the integral elastomeric layer or film is typically in the range of 25 mils-30 mils (625xcexc-750xcexc). The elastomeric film is preferably coextruded over the HDPE sheet which is typically 180 mils-250 mils (4.5 mm-6.2 mm) in thickness. The elastomeric film layer can alternatively be applied to the HDPE sheet by lamination, by an adhesive, or by heat application.
According to the Dresen et al. scheme, an increased frictional force is achieved primarily by the xe2x80x9cplowing effectxe2x80x9d or embedding effect of harder cargo pieces pressing into the softer continuous elastomer film layer. This antislip plowing effect is attributable to the continuous film or layer of the softer elastomer. A disadvantage of the Dresen et al. scheme however, is that the frictional force attributable to the integral elastomeric layer is reduced, and the layer also becomes slippery, when it is wet. Another disadvantage of the Dresen et al. method is the increase in cost of adding and applying the integral and continuous film layer of elastomeric material by coextrusion, lamination, adhesion, or heat application. The elastomer film of Dresen et al. generally extends across the entire substrate plastic sheet. The Dresen et al. film also does not allow different antislip characteristics to be achieved at different locations, namely the film has a uniform antislip characteristic over the entire area to which it is applied.
In view of the above discussion, it is an object of the present invention to provide thermoplastic products that have more effective antislip surface properties in a manner that is more economical and adaptable to a greater variety of products. More specifically, the invention aims to avoid the disadvantages in the prior art, namely to provide a thermoplastic product having an antislip surface that retains its antislip surface properties even when wet, that avoids the use of a continuous film of antislip material, and that provides good adhesion, durability and wearability of the antislip material on the thermoplastic base material of the product.
It is a further object of the invention to provide methods of imparting antislip properties to thermoplastic surfaces, which increase frictional forces and the coefficient of friction for wet as well as dry surfaces. The different methods are respectively suitable for achieving different characteristics of the antislip surface, and for application to different products formed by different techniques. A first method is especially suited to form an antislip surface be fore and during the thermoforming of the product, and a second method is particularly suited to provide an antislip surface on a product that has been previously molded or otherwise formed.
Another object of the invention is to increase the practical coefficient of friction and frictional force on thermoplastic surfaces by means of increased three dimensional macroscopic surface relief, asperities, and roughness. This is to be accomplished with hard and tough antislip materials preferably having viscoelastic or elastomeric properties and a hardness that is not brittle or sharp. It is thus an object of the invention to specify particular materials and combinations of materials that can be applied as preferably discrete individual asperities onto a thermoplastic base material, and especially HDPE, with sufficient adhesion to avoid abrasive removal thereof.
A further object of the invention is to impart antislip properties and increase the coefficient of friction on thermoplastic surfaces by the composition of the surface with reduced materials and expense and without the cost of handling and applying continuous or integral film layers on a substrate plastic surface. Furthermore, the antislip surface can easily be applied very selectively to only portions of the thermoformed plastic surface in any shape or configuration while leaving other areas untreated. Also, the invention aims to allow the resulting coefficient of friction to be adjusted or tailored to the particular application, even with different coefficients of friction on different surface areas of the product.
In order to achieve the above objects, the invention provides thermoplastic products having antislip surfaces formed by a plurality of asperities comprising an antislip polymer applied and durably bonded onto a selected surface area of a thermoplastic base member. The asperities are primarily separate from one another so as to leave the thermoplastic base member exposed between the asperities and so as not to form a continuous film of the antislip polymer. The asperities provide a relatively increased coefficient of friction of the selected surface area in comparison to a base coefficient of friction of the thermoplastic base member without the asperities thereon. The invention especially provides a liner for lining a cargo area of a vehicle. The liner includes a liner floor comprising a thermoplastic polymer, and the antislip asperities applied and durably bonded onto an upper surface thereof. Additional asperities may be provided on the bottom surface of the liner, to enhance the frictional gripping of the liner on the vehicle cargo area floor, especially for a mat-type liner. The invention also applies to a floor mat having asperities on a top surface and on a bottom surface thereof. The asperities provide a coefficient of friction that is preferably at least 1.5, or especially at least 2 times the base coefficient of friction.
The present thermoplastic products may especially be made of a base material of high density polyethylene, such as a sheet of HDPE that may be formed or molded by any known process. Other plastics for the base material include polypropylene, polystyrene, ABS plastic, polyvinyl chloride, TPO/TPR, acrylic, PET, polycarbonate, and nylon, for example. The base material may also be a multi-layer material, for example including a layer of another polymer coextruded or laminated over a polyethylene base layer, with the antislip asperities applied onto the coextruded or laminated layer. The antislip material may particularly be a thermosetting polymer, and preferably a polyurea based polymer, and most preferably a hybrid polyure-apolyurethane polymer formed from a polyurea prepolymer and an isocyanate.
The antislip surface treatment may be applied to the present products by at least two different types of methods. U.S. Pat. No. 5,648,031 (Sturtevant et al.), which issued from the parent of the present application, describes a first method for applying the antislip surface treatment, which generally involves applying droplets of the antislip polymer onto the thermoplastic base material, for example by spraying, and then heating and thermoforming the base sheet with the antislip material droplets thereon. The thermoforming process preferably hardens the droplets of antislip material into tough macroscopic asperities that cause a relatively higher coefficient of friction on the treated surface area, and durably bonds the antislip material onto the thermoplastic base material. Furthermore, the application of the antislip droplets is limited, so that the resulting bumps preferably cover less than 50% of the treated surface area, i.e. leave more than 50% of the thermoplastic base material exposed. Also, according to the first method, the antislip polymer is preferably selected so that the resulting antislip bumps or asperities have a hardness that is greater than the hardness of the thermoplastic base material.
The above discussed first method of manufacturing the present thermoplastic products having antislip surfaces was the original method determined to achieve good product results, e.g. frictional characteristics and bonding of the asperities onto the base sheet. Through further study, the inventors have determined that a second method may also be effectively used to achieve good product results, with a different or broader range of applicability. The second or alternative method does not require the thermoplastic product to be thermoformed after the antislip material is applied onto a surface thereof. Instead, the thermoplastic product can be formed by any conventional forming or molding process as a prior or initial step, and subsequently the antislip surface treatment is applied onto the previously formed product and then heat treated, as will be discussed below.
As such, the second method can be used for making a broader range of products, i.e. products that are not thermoformed, but instead are formed by injection molding, blow molding, compression molding, rotational molding, etc. The second method can also be applied to products that are thermoformed. The second method is particularly useful in applications in which it is desired to apply an antislip surface treatment on a surface that necessarily faces or contacts the mold during the forming operation. Generally, either or both of the methods are useful for applying an antislip surface onto vehicle cargo bedliners, such as cargo area liners for pickup trucks, sport utility vehicles, passenger cars, agricultural vehicles and the like, vehicle floor mats, flooring steps and deck surfaces of swimming pools and spas, bathtubs, shower floors and the like, walking surfaces that are subject to wet conditions, such as mezzanines, marine decks and docks, step plates, ramp and conveyor surfaces, plastic lumber fabricated structures, and the like. Another important area of application is for returnable packaging, shipping, and goods handling products, for example to apply an antislip surface onto the decks, fork surfaces, and footprint surfaces of pallets, and trays, spacers, totes, bins, and slip sheets used for shipping and handling various goods. This provides a significant advantage over the prior art use of rubber rims, lips or feet which are independent separable components. The second method may further particularly be used for applying antislip surfaces to recreational and sporting equipment surfaces, such as watercraft deck surfaces, playground equipment, exercise equipment, sleds, shed floors, and the like, landscaping and geomembrane sheets and liners, including landfill friction sheet and landscape reshaping restraints, and floor coverings such as mats and runners.
Furthermore, the inventors have found that other limitations originally discussed in U.S. Pat. No. 5,648,031 are not strictly necessary. For example, the surface area coverage ratio of the bumps or asperities of antislip material does not need to be limited to a range less than 50%. Instead, the antislip material may be applied to cover a surface area percentage of 60 or 70%, for example, limited only so as to avoid forming a continuous film or layer of the antislip material. This greater range of coverage ratios of the antislip material pertains to both the first and the second methods. Also, the size of the bumps or asperities produced according to the second method is generally preferred to be slightly larger, on average, than the size of the bumps or asperities produced according to the first method.
It has also been discovered that the antislip bump material does not have to be harder than the base material, but instead can be designed and processed to be softer or to have about the same hardness as the base material, depending on the particular product application. For example, harder bumps are preferably used if the intended application involves softer goods, such as cardboard cartons, being placed onto the antislip surface, and softer bumps are preferably used if the intended application involves harder goods, such as steel parts, being placed onto the antislip surface. Moreover, the relative hardness of the asperities depends on the hardness of the base material, whereby for example, HDPE may even have a hardness up to 90 Shore D. This feature applies to both the original or first method and the alternative or second method, but the bump hardness is particularly adjustable by controlling the processing conditions in the second method. In other words, the thermoforming process applied during the course of the first or original method generally results in antislip asperities that are harder than the base material, while the hardness of the asperities is especially adjustable or variable by the second method.
Further studies carried out after the time of filing the parent application resulting in U.S. Pat. No. 5,648,031 have shown that there are two important common or unifying features between the first method and the second method, for achieving good frictional characteristics and good adhesion of the asperities onto the thermoplastic base material. The first important common feature is the particular polymer materials that can be used for providing a durable antislip surface on thermoplastic products, and especially high density polyethylene products. The second common feature is the need of a heat treatment of some kind to enhance the bonding of the asperities onto the thermoplastic base material.
Particular preferred aspects of the original or first process according to the invention will now be discussed. As will be apparent, many of these aspects also pertain to the second or alternative method. The original process is for selectively forming an antislip surface on a product to be thermoformed from a plastic sheet. The plastic sheet may be formed of any suitable thermoplastic polymer material such as polyethylene, ABS, synthetic rubber, thermoplastic polymers such as polypropylene, and e.g. other polyolefin thermoplastic polymers. Typically the original method proceeds by cleaning the surface of the plastic sheet for removing oils and other contaminants and preparing a good bonding surface. The method further proceeds by masking the plastic sheet for exposing selected surface areas to be treated with the antislip surface and covering other areas not to be treated. In some applications the cleaning and masking steps are not required.
An embodiment of the original method includes atomizing into droplets a sprayable polymer, preferably a thermosetting elastomeric polymer or other polymer having a viscoelastic or elastomeric component and a relatively rapid reaction time. Other polymers which harden to a hard tough plastic material without brittleness or sharpness can also be used as hereafter described. The original method proceeds by flash spraying a mist of the atomized droplets onto the masked plastic sheet, or otherwise forming and applying the droplets to the selected exposed area of the plastic sheet. The droplets are deposited so that they form a stippled pattern of primarily separate bumps or pimples across the treated surface area. The original method also preferably includes controlling the time duration of the flash spraying or otherwise controlling the application of droplets so that the bumps or pimples cover or occupy substantially less than 50% of the treated surface area leaving the underlying plastic sheet substantially exposed between the droplets.
By reason of the masking step, the stippled pattern of primarily separate droplets or pimples can be selectively applied over only a portion of the HDPE sheet in any desired shape or area configuration. The treated surface area can thereby be limited for example to a cargo carrying surface or only a portion of the cargo carrying surface. By way of example, only a fraction of the cargo carrying surface, such as one half of the surface, can be treated for a higher coefficient of friction while the other half remains as an HDPE surface with lower coefficient of friction. Thus the right half of the truck bed liner surface may be treated to provide an antislip surface for cargo containers while the left half permits sliding of the objects for loading and unloading. As a further alternative, the stippled pattern of primarily separate droplets need not be selectively applied and the masking step can be eliminated. For example, the entire HDPE sheet can be treated with the stippled pattern of separate droplets to impart antislip properties over all the surfaces of the truck bed liner or other thermoformed product.
As a next step of the original method, the droplets forming the stippled pattern are at least partially hardened, dried and solidified in an initial drying step. The plastic sheet is then thermoformed according to standard thermoforming procedures at thermoforming temperatures to produce the product. The thermo forming process causes intimate bonding of the droplets to the plastic sheet and relative hardening of the droplets to form three dimensional macroscopic asperities having an enhanced relief and surface roughness, causing a relatively higher coefficient of friction over the treated surface area.
Thus, a feature of the method is that the process steps produce an antislip surface across the treated surface area with enhanced relief in the form of three dimensional macroscopic asperities and surface roughness. These macroscopic asperities of the antislip surface engage the peaks, valleys and surface relief of objects placed on the antislip surface by interlocking of asperities and macroscopic roughness on the two surfaces. As a result, the coefficient of friction and frictional force is substantially increased relative to an untreated surface, and is not substantially reduced when the surface is subsequently made wet. The enhanced surface roughness and macroscopic asperities are achieved by several coacting effects of the interacting process steps.
First it has been found that the thermoforming temperatures and process cause an intimate physical or chemical bond between the thermosetting elastomeric polymer droplets and the plastic sheet typically composed of high density polyethylene. The bonding between the droplets and the HDPE sheet is apparently as strong as the bonding of polyethylene to polyethylene, and the droplets cannot be scraped off without cutting through the bonded materials. This bonding between the droplets and the HDPE sheet is enhanced by mixing with the thermosetting elastomer a small amount of an adhesion promoter which bonds or cross links both to the thermosetting elastomer and to the HDPE sheet.
Second it appears that the thermo forming temperatures cause a relative hardening and post-cure thermosetting of the thermoset elastomeric polymer that was sprayed in droplet form and initially hardened, dried and cured on the exposed surf ace areas of the plastic sheet. While the thermosetting elastomer retains some elastomeric properties, it is further hardened as compared to its condition prior to heat curing. The increased coefficient of friction and frictional force effect is therefore achieved primarily not by elastomeric properties but by the relative hardness and the surface texture of the droplets forming macroscopic asperities across the treated surface area. Other sprayable or liquid polymers that are formable into droplets and hardened by the thermoforming process can also be used as hereafter described.
Third, the projecting asperities, pimples, or projections formed by the droplets bonded and hardened across the treated surface area are enhanced in relief by an effect referred to by the inventors as the xe2x80x9cmoth effectxe2x80x9d. Thermoforming heat is generally applied to both sides of the plastic sheet by a variety of heater arrangements such as infra-red radiant heaters and catalytic gas burners for example. Electrical heating elements with blowers are also typically used. The inventors have noted that if a moth falls on the plastic sheet before or during heating of the sheet, then a corresponding moth-shaped outline or plateau rising slightly above the surrounding area of the sheet will be formed during the thermoforming process. It is believed this effect is due to xe2x80x9cshadingxe2x80x9d or xe2x80x9cscreeningxe2x80x9d provided by the moth, which produces a temperature differential. Thereby, the higher temperature areas surrounding the shaded outline of the moth are subject to differential thermoforming and thinning, so that these surrounding areas are more greatly thinned and pulled away from the moth shaded area. This causes a slightly differential thickness or enhanced surface relief according to the pattern of differential temperatures.
Similarly it has been observed that the droplets forming a stippled pattern across the treated surface area shade or screen the spots under the droplets from the heat sources used to bring the plastic sheet to thermoforming temperatures. The stippled pattern of droplets thus produces a corresponding stippled pattern of shading and differential temperatures across the surface area to be treated. Furthermore, the droplet material mechanically reinforces the plastic sheet to resist vacuum forming at the locations under the droplets. During thermoforming, the areas of the plastic sheet surrounding the droplets are vacuum formed, stretched and pulled to a lesser thickness than the shaded areas under the droplets. In other words, the covered or shaded areas directly beneath the droplets are not heated and softened as much as the exposed or non-shaded areas, so that the softer exposed areas of the base sheet are more greatly stretched and thinned than the shaded or covered areas during the thermoforming process. This effect enhances the relief, projection and elevation of the macroscopic asperities, and the roughness across the treated surface area. For example, it has been observed that the height of the asperities on a finished thermoformed product is in a range from more than one up to two times the height of the droplet projections on a sheet before or without thermoforming.
Finally, some of the droplets atomized from the viscous thermosetting elastomeric polymer entrap air or other gases. The entrapped air or gases may burst or explode through the top of the droplet during thermoforming thereby causing a so-called xe2x80x9cvolcano effectxe2x80x9d that forms rough edges around resulting craters or blow-holes in the droplets to increase the coefficient of friction. The increased roughness is achieved on the hard, tough pimples without brittleness or sharpness. Moreover, it has been reported that the inventive asperities in general, and particularly the rough crater edges formed by the volcano effect cause a hook-and-loop fastening effect when placed in contact with carpet. This is very advantageous for providing an antislip surface on the underside of floor mats and the like.
In the preferred example embodiment the original process includes cleaning the surface of the plastic sheet by flame treatment for burning off contaminants and preparing a good bonding surface, applying the droplets, at least partially hardening and solidifying the droplets forming the stippled pattern over the treated surface area, and thermoforming the HDPE sheet to form the final product in the thermoforming temperature range of 250xc2x0 F.-550xc2x0 F. and preferably in the range of 370xc2x0 F.-400xc2x0 F. The step of initially drying or hardening and solidifying the droplets before thermoforming can be accelerated by heating. The time duration of the flash spraying may be controlled to an interval for example down to a fraction of a second through a spray head at a relatively high pressure, for example in the range of 1,000-2,000 psi. Typically the flash spraying is from a spray head at a distance from the HDPE sheet in the range of 2.5xe2x80x2-4.5xe2x80x2 (0.75 m-1.35 m).
According to the preferred examples, the atomizable or sprayable polymer is selected to form hard and tough pimples durably bonded over the selected antislip surface after thermoforming. The durably bonded hard and tough droplets are not brittle or sharp. A variety of starting polymers can be used for the atomizable, sprayable polymer designed to achieve the desired characteristics of hardness, toughness, and strong bonding and without impact brittleness or sharpness. The atomizable, sprayable polymer is preferably a thermosetting elastomeric polymer such as, for example, polyurea prepolymers and polymers, hybrid prepolymers and polymers such as hybrid polyurea polyurethane polymers in which the polyurea bonds predominate, and other polyether resins generally including epoxy resins, for example. High temperature tolerant thermoplastics such as polyester resins, as well as HDPE and PVC are also suitable.
It is noted that both thermosetting polymers and thermoplastic polymers as well as combinations of thermoset and thermoplastic can be used for the sprayable polymer. The sprayable polymer is selected to provide the hard, tough pimples on the stippled surface. The polymer or polymer mixture is selected to provide toughness and a hardness of the asperities that is preferably greater than the hardness of the underlying thermoplastic sheet but below brittleness and sharpness. Alternatively, and especially according to the second method, the asperities may have the same or a lower hardness than the base material.
If thermoplastic polymers, such as styrene, are to be used for the droplets, then they are first made sprayable by mixing with a suitable solvent. After being sprayed on, the droplets are initially dried and hardened by evaporating the solvent. Alternatively, lower viscosity thermoplastics can be made sprayable by heating the bulk thermoplastic polymer material to a suitable temperature at which droplets can be formed for misting or spraying. Initial drying and hardening is achieved by lowering the temperature to a suitable hardening temperature. For the thermosetting resins, sprayability is achieved as described above by mixing and spraying the reacting components. Initial drying and hardening is achieved by the reaction of the components. It is also noted that the selection of the sprayable polymer or polymer mixture increases the friction on the stippled surface not only by the surface profile of hard macroscopic asperities but also by the composition of the surface material. For example the polyurea polymer asperities as well as asperities of other thermoset and thermoplastic polymers provide greater interaction with any object passing over the surfaces than does the HDPE surface of the underlying sheet.
The polymer droplets can be formed and applied on the selected surface area by methods other than misting and spraying. For example, the droplets can be formed and applied by a roller containing the polymer in sprayable or liquid form. The roller has pin holes or ejector holes through which droplets are dispensed as the roller passes or turns over the selected surface area. The pin holes or ejector holes are arranged with the appropriate density for dispensing droplets on preferably less than 50% of the surface area. Alternatively the substrate sheet can pass under a droplet dispenser having a reservoir of the polymer in sprayable or liquid form. The reservoir is formed e.g. with one or more rows of pinholes for forming and dispensing droplets on the selected surface area in the desired density covering less than 50% of the surface area according to preferred examples. As a further alternative, a plurality of nozzles may extrude plural continuous or intermittent strings, stripes or ribbons of the polymer onto the base sheet.
As used in the specification and claims, the adjective xe2x80x9csprayablexe2x80x9d does not necessarily imply that the polymer is sprayed but rather that the polymer is prepared in a sprayable condition, such as liquid form, capable of formation into droplets. The sprayable polymer may then be formed into droplets and dispensed by misting and spraying or by xe2x80x9cdrippingxe2x80x9d into droplets falling on the selected surface area or applied onto the selected surface area by a roller or reservoir dispenser or other droplet applying means, such as extruder nozzles extruding strips or stripes of the antislip material.
Appropriate additives and agents may be required to achieve the desired characteristics. An adhesion promoter can be added to the thermosetting elastomeric polymer, thermoplastic polymer, or polymer mixture, in the range of for example 1%-7% by weight and preferably about 5% by weight of the mixture. A suitable adhesion promoter is for example a cross linking Melamine resin.
The preferred size range of the droplets of the viscous thermosetting elastomeric polymer, thermoplastic polymer, or polymer mixture for the original method is 2 mils-30 mils (50xcexc-750xcexc) in diameter. Larger size droplets can also be used. After thermoforming, the bumps or asperities forming the stippled pattern of pimples across the treated surface area portion are also substantially in the size range of 2 mils-30 mils (50xcexc-750xcexc) although larger size bumps are also suitable.
Particular preferred aspects of the second or alternative process according to the invention will now be discussed. Most of the features and aspects relating to the original process also pertain to the second or alternative process. For example, the possible base materials and the antislip materials applied thereon relating to the first process can also be applied according to the second process. The process steps relating to cleaning the base material, masking the base material, and applying the antislip material, preferably by spraying droplets of the antislip material onto the base material, also pertain to the alternative method. However, as discussed above, the starting base material for the second alternative method is a product that has already been formed or molded essentially into its finished form, before the antislip treatment is carried out.
The second alternative process generally includes a preliminary or prior step of molding or forming and then trimming, in the usual or conventional manner, the article that is to be treated with the nonslip surface. The thermoplastic article is then cleaned to remove any mold release compound, or other surface contaminants such as dirt, oils, fingerprints, or the like. Next, the surface of the article to be treated is oxidized or polarized by a flame treatment, electrical corona discharge, chemical treatment, for example with chromic acid, the so-called USM process, or a suppressed spark method. Thereafter, the antislip polymer material, preferably a two-component thermosetting polymer, is applied onto the oxidized surface, preferably by spraying. Finally, the treated surface with the antislip material droplets or bumps thereon is subjected to a heat treatment preferably in the range of 100 to 200xc2x0 F., and especially above 120xc2x0 F., for a time in the range of 30 to 60 seconds, in order to harden the antislip polymer material and durably bond it onto the base material.
Due to the lack of a thermoforming step after the antislip material droplets are applied, the second or alternative method does not provide the above described xe2x80x9cmoth effectxe2x80x9d and generally does not provide the above described xe2x80x9cvolcano effectxe2x80x9d. Thus, the resulting bumps or asperities generally do not have the enhanced roughness provided by the cratering caused by the volcano effect, and do not have the slight enhanced relief caused by the moth effect.
The invention is particularly applicable to truck bed liners manufactured from an HDPE plastic sheet or other thermoplastic sheet. The plastic sheet is masked to expose a portion of the cargo carrying surface of the truck bed liner for treatment with an antislip surface. Other applications for the invention include slip sheets, pallets, and other industrial applications as well as games and recreational products such as xe2x80x9cslide barsxe2x80x9d and ski devices. Other applications include industrial, commercial, domestic, and recreational products. In the case of slip sheets and other substantially flat surface products, thermoforming can take place in a mold with a shallow basin well according to the first method, or the second method can be used without thermoforming. An undersize sheet can be used so that the flat surface product portion is pulled into the shallow basin to achieve the differential thinning effect with enhanced asperities in the first method. Such thermoforming achieves the enhanced and increased three dimensional relief provided by the macroscopic droplet asperities rising above the surrounding surface of the vacuum thinned plastic sheet in the first method. The thermoforming of the first method, or the heat treatment of the second method, also completes a secure bonding between the droplets and the HDPE substrate and causes a relative hardening of the thermoset polymer droplets.